1. Field of the Invention
The present invention relates to a Multimedia Broadcast/Multicast Service (MBMS) of a mobile communication system. More particularly, the present invention relates to a hard handover method and apparatus for a User Equipment (UE) using Frequency Layer Convergence (FLC) with a dedicated channel.
2. Description of the Related Art
Currently, mobile communication systems using a wideband Code Division Multiple Access (CDMA) scheme are being developed for packet service communication for the transmission of mass storage data and multimedia broadcast/communication for the transmission of multimedia services other than conventional voice communication. Accordingly, in order to support multimedia broadcast/multimedia communication (MBMS), an MBMS service capable of providing a service to a plurality of user equipments (UEs) from one or a plurality of multimedia data sources has been discussed.
The term “MBMS service” typically refers to a service for transmitting the same multimedia data to a plurality of receivers through a wireless network. Herein, the receivers share one radio channel, so that radio transmission resources can be efficiently used. This type of MBMS service is capable of supporting multimedia transmission of realtime images and voices, still images, texts, and so on, while simultaneously providing voice and image data according to the type of multimedia transmission. This requires a large amount of resources. In an MBMS service, because the same data must be transmitted to a plurality of cells including users, a Point-to-Point (PtP) connection or a Point-to-Multiple (PtM) connection is used according to the number of users included in each cell.
A conventional MBMS system supports FLC technology capable of allocating a preferred frequency to each available MBMS service and allowing UEs using the MBMS services to receive a desired MBMS service through the preferred frequency as much as possible. That is, a FLC of a MBMS corresponds to a method for moving a UE to a preferred frequency band according to each service in order to provide a service through only one frequency according to each available service, instead of providing one service through overlapping frequencies in the same service area.
FIG. 1 is a diagram illustrating a conventional MBMS FLC operation. FIG. 1 shows a cell structure in which cells using different frequencies f1 and f2 overlap geographically in a 3rd Generation Project Partnership (3GPP) system which corresponds to a standard of a 3rd generation asynchronous mobile communication network scheme based on a Global System for Mobile Communication network (GSM) and a General Packet Radio Services (GPRS). Further, one cell comprised of one frequency or a layer of continuous cells will be referred to as a layer.
In FIG. 1, reference numbers 101 and 102 represent layers comprised of the frequency f1, respectively, and reference numbers 111 and 112 represent layers comprised of the frequency f2, respectively. An FLC moves a UE to a corresponding frequency band according to each service in order to provide a service via one frequency according to each available service, instead of providing one service through all cells of the frequencies f1 and f2 that overlap in the same service area as illustrated in FIG. 1.
When a layer having a frequency f2 allocated for a MBMS service, which a UE 121 has joined, the UE 121 which currently uses the frequency f1 which overlaps in the same service or coverage area as frequency f2, the UE 121 must move to the cell having the frequency f2 in order to continue to receive the MBMS service when a session of the MBMS service starts and while the session is in progress. A Radio Network Controller (RNC) informs UEs using the frequency f1 that they must move to the cell of the frequency f2 when the session of the MBMS service starts and while the session is in progress.
In the MBMS service, the cell of the frequency f1 will be referred to as a Preferred Frequency Layer (PL) and the cell of the frequency f2 will be referred to as a Non-Preferred Frequency Layer (NPL). It should be noted that the term NPL does not always denote that the MBMS service is unavailable. That is, the MBMS service may be provided through the NPL for a UE remaining in the NPL by a Radio Resource Management (RRM) in spite of a FLC method.
Typically, the MBMS service has one layer, but may have different layers according to areas. For example, in one area, a layer of the frequency f2 may be a PL. However, in another area, a layer of the frequency f1 may be a PL. In the MBMS service, a PL is determined by a cell structure policy of a system operator and the radio resource availability of a currently used cell. However, a conventional FLC structure representing a frequency layer allocated to the MBMS service is applied until one session of the MBMS service starts and ends.
FIG. 2 is a flow diagram illustrating a conventional hard handover process of a UE. Specifically, FIG. 2 shows an operation of a CELL_DCH UE that receives a service through a Dedicated Channel (DCH). A reference number 201 represents the CELL_DCH UE and a reference number 202 represents a Serving RNC (SRNC) of the UE 201.
Referring to FIG. 2, in step 211, the UE 201 transmits measurement reports for a downlink to the SRNC 202 by means of an uplink message. The downlink measurement report includes inter-frequency measurement report values, and may be classified as a periodic report and an event-triggered report. In step 221, the SRNC 202 receives the measurement report and determines if the hard handover for the UE 201 must be performed based on the inter-frequency measurement reports and information on loads of cells. If the SRNC 202 selects a hard handover for the UE 201 in step 221, the SRNC 202 directs the UE 201 to perform the hard handover through a physical channel reconfiguration message in step 231.
FIG. 3 is a diagram illustrating a conventional procedure by which a MBMS FLC is applied to a CELL_DCH UE. A reference number 301 represents a cell having the frequency f1, a reference number 302 represents a cell having the frequency f2, a reference number 311 represents the CELL_DCH UE receiving a MBMS service, and a reference number 321 represents a SRNC for controlling the UE 311. Before the MBMS service starts, the UE 311 is located in the cell having the frequency f1.
Referring to FIG. 3, a RNC 321 receives a session start message for the MBMS service (service A), which the UE 311 has joined, from a Serving GPRS Support Node (SGSN), and transmits an ID “A” of the MBMS service, a preferred frequency “f2” of the service A, MBMS offset and session ID information to the UE 311 through a dedicated notification message, in the first step. This allows UEs intended to receive the service A, as opposed to UEs located in cells using frequencies other than the frequency f2, to move to the frequency f2.
When the UE 311 has received the dedicated notification message while receiving a different service (service B) employing the frequency f1 as a preferred frequency band in the second step, the UE 311 simultaneously cannot receive the service B currently being provided and the service A to be provided. This is because the preferred frequency bands of the services A and B are different from each other. Accordingly, the UE 311 selects the service having the highest priority between the services A and B in the third step. This service selection procedure may be performed through mutual information exchange between the UE and a user or applications.
However, the RNC 321 does not know the result of the service selection. Accordingly, even when the UE 311 has selected the service B, the RNC 321 may determine a hard handover to the frequency f2 is necessary in order to allow the UE 311 to receive the service A in a state in which the RNC 321 does not know that the UE 311 has selected the service B, in the fourth step. When the RNC 321 selects the hard handover in the fourth step, the RNC 321 directs the UE 311 to perform the hard handover through a physical channel reconfiguration message in the fifth step. Accordingly, the UE 311 performs the hard handover to the frequency f2 without knowing that the hard handover is a frequency shift for_reception of the service A. As a result, the UE 311 does not receive the desired service B.
In the conventional FLC method operating as described above, when the CELL_DCH UE is shifted to a corresponding PL for reception of the MBMS service, an erroneous hard handover may be performed because the SRNC does not know whether the UE has received the service. Further, because the CELL_DCH UE does not indicate that the physical channel reconfiguration message, which is a hard handover indication message received from the SRNC, is for a PL shift of a specific MBMS service or for measurement reports and cell load control, the UE may perform an erroneous hard handover.